1. Field of the Invention
The present invention relates to a resonant converting circuit and a resonant controller, and more particularly relates to a resonant converting circuit and a resonant controller with a function of resonance deviation-free.
2. Description of Related Art
A resonant converting circuit has an advantage of high conversion efficiency by zero voltage switching or zero current switching, particularly a half-bridge LLC converting circuit, and is used in more and more wide field.
FIG. 1 is a schematic diagram of a relationship between a gain and an operating frequency of a half-bridge LLC converting circuit, wherein a frequency f1 is a frequency corresponding to a full load rating and a frequency f2 is a frequency corresponding to a peak of a resonance curve. The frequency f2 divides the resonance curve into operating regions of zero voltage switching (ZVS) and zero current switching (ZCS). The gain is decreased while the frequency increases in the operating region of zero voltage switching and increased while the operating frequency increases in the operating region of zero current switching. The half-bridge LLC converting circuit is usually designed to operate in the region of zero voltage switching for decreasing switching loss. If the half-bridge LLC converting circuit enters the region of zero current switching, the following serious results occur. First, the half-bridge LLC converting circuit losses the control of an output voltage due to that a negative feedback is changed into a positive feedback. Second, the switch loss of the full-bridge LLC converting circuit becomes larger.
In the conventional arts, the protection method for avoiding the half-bridge LLC converting circuit operating in the region of zero current switching is increasing operation frequency or limiting the minimum duty cycle. However, whichever method mentioned above can't ensure that the half-bridge LLC converting circuit never operates in the region of zero current switching under all conditions, such as: the circuit operating abnormally, the parameters of components being wrongly set.